2006 Water Comprehensive Plan - City of Bellevue

Pressure ZonesPressure zones are determined by evaluating ground elevation as it relates to available hydraulicgradient. The static pressure at the lowest elevation should generally not be greater than100 psi. The static pressure at the highest elevation of the zone should be sufficient to ensurethat at peak hour demand conditions the pressure is not less than 30 psi. Individual pressurecontrol devices are required on service lines where pressures exceed 80 psi, per the InternationalPlumbing Code.Ideally, a minimum of two supply sources should serve each pressure zone to ensure thatminimum pressures are maintained and to meet system supply and fire-fighting requirementswhen one source is out of service. Bellevue's design criteria limit single supply areas to lessthan 250 homes.Existing street configuration and location of pressure reducing valves (PRVs) and supply pointsmust be considered when recommending changes to the existing pressure zones. Exceptions tothe static pressure standards are made when streets and mains do not conveniently follow theideal ground contours. Bellevue's water system was developed by integrating a number ofseparate water districts to form the current system. Combining these districts into one systemhas required compromises regarding service pressures. Major improvements required todevelop a uniform pressure range throughout the city are more costly than the derived benefit.Ideally, the number of pressure zones created should be kept to a minimum. Wherever pressurezones are created, the system becomes fragmented, and the water conveyance capacity can beaffected because distribution mains cannot be interconnected at pressure zone boundaries.PRVs are mechanical devices requiring maintenance and are subject to failure.Pump StationsA minimum of two pumps are required at each pump station to provide flexibility and systemredundancy. The City of Bellevue Utilities Department Engineering Standards require thatpumps in supply pump stations be sized so that the station can meet MDD design conditionswhen the largest pump is out of service. Pumps serving pressure systems must consideravailable storage so that the storage and pump capacity can provide peak hour demand as wellas fire flow demand. The total capacity of the pumps should be 25 to 50 percent greater than therequired capacity of the pump station in order to provide storage replenishment.To increase emergency reliability, at least one pump in each pump station should be suppliedwith onsite standby power or have the capability to connect to a portable power supply. Thiscapability allows some emergency supply capacity, even during a general poweroutage. Where no direct storage is available to an area with a single supply source, onsite powergeneration is required.4-18